The present invention relates to energy generation systems and, more particularly, to a natural convection boiling water reactor where the reactor can be a fission reactor. More specifically, the present invention applies to boiling water reactors which utilize a chimney for the augmentation of the coolant circulation flow, and which utilize free-surface steam separation for the extraction of the steam phase used to deliver energy from the recirculating water phase.
In a boiling water reactor, heat generated by a radioactive core can be used to boil water to produce steam, which in turn can be used to drive a turbine to generate electricity. Natural convection boiling water reactors limit complexity by dispensing with the need for pumping water within the reactor vessel. The nuclear core which generates heat is immersed in water within the reactor vessel. Water circulated up through the core and a chimney above the core is at least partially converted to steam which forms a relatively low pressure head above the core. Water recirculates down a downcomer annulus between the reactor vessel and the chimney and core. The water in the downcomer is denser than the steam and water mixture in the core and chimney region. The difference in density forces circulation up through the core and chimney and down through the downcomer.
The chimney directs the steam water mixture vertically from the core. This vertical direction is best effected where the chimney includes multiple vertical sections, each of which serves as a chimney for the portion of the core directly below it. Confining the steam path in this way helps maintain a head of steam above the core, facilitating water circulation.
The steam emerging from the chimney top rises through the water in the reactor vessel and exits through a steam nozzle at the vessel top. Typically, a flat annular array of dryers is disposed near the vessel top to trap any water being carried by the steam, and return trapped water to the recirculation fluid. Otherwise, water carried by the steam would limit the efficiency with which the steam can drive a turbine or other energy conversion device. Since there is a net loss of water+steam from the vessel through the exit port, means are provided to replenish the water in the vessel. This is normally accomplished by returning condensation from the turbine using a fluid handling system, including a feed pump which pumps water through a feedwater sparger which distributes subcooled return water around the downcomer.
Two phenomena adversely affect the performance of a natural convection boiling water reactor. "Carryover" refers to water carried in the flow of steam from the vessel, while "carryunder" refers to steam carried in the flow of water recirculating within the vessel and through the core. Carryover can damage the turbine and adversely affects the efficiency with which a turbine can be driven.
Carryunder comprises steam bubbles which have a high thermal energy per unit mass so that they can impair the subcooling provided through the feedwater sparger. The result is a higher water temperature at the core entrance, and more rapid boiling of the recirculation fluid as it flows up through the core. The more rapid boiling increases the steam voids within the core. The larger voids result in higher irreversible pressure drops through the fuel bundle than would be the case with smaller voids. This effect is amplified, since the larger voids tend to choke recirculation flow, despite a higher driving head. These irreversible head losses can be compensated in the design stage by providing greater chimney height, but this results in a bigger vessel and significantly greater reactor costs.
In addition, the larger voids adversely affect core stability, as the stability-decay ratio is dependent on the proportion of two-phase pressure drop to single-phase pressure drop. This lower stability must be addressed by limiting the power production level below what might otherwise be obtainable. Furthermore, the larger voids create a negative reactivity, requiring the control rods to be withdrawn farther from the core. This reduces the opportunity to achieve long fuel burnups for a given initial core enrichment.
Carryover and carryunder both result from the inadequate separation of steam and water, generally above the chimney. Given sufficient time, the different densities of steam and water would allow adequate separation. However, water is swept along with the upward steam flow and steam is swept along with the radially outward and then downward water flow too rapidly for complete separation.
The time available for water and steam to separate can be increased either by reducing flow rates or by increasing flow distances. It is counterproductive to reduce flow rates. The steam flow rate directly impacts turbine output, while water flow impacts core void size and thus the efficiency with which neutrons generate heat. As an alternative, the reactor vessel can be made larger to accommodate longer flow paths within the vessel. However, enlarging the vessel not only increases the cost of the vessel, but requires geometrically larger versions of the multiple containment systems provided for a reactor vessel. Larger containment systems require more materials, more maintenance, and greater potential exposure of personnel to nuclear radiation or contaminants.
What is needed is a natural convection boiling water reactor system which reduces carryunder and carryover without requiring a larger reactor vessel and without reducing the flow of steam from the vessel or water through the core. In addition, the increased efficiency provided by such a reactor system should be achieved without substantial costs in terms of size, complexity or safety.